The present invention broadly relates to the papermaking art, and, more specifically, concerns a new and improved construction of a double filter papermaking machine, also sometimes referred to in the art as a twin-wire type of papermaking machine.
In particular, the present invention relates to a double filter or twin-wire papermaking machine comprising two endless filters, typically in the form of wires or felts, which serve to guide a fiber web or fleece located therebetween along part of the surface of a dewatering cylinder located below the filters. In front of the cylinder there is located a linear section of the inner filter which is situated closer to the cylinder. This linear section of the inner filter deviates from the horizontal direction through a maximum of 45.degree.. A headbox or equivalent structure is provided for the inner filter for the infeed of the stock suspension or the like. Between the linear section of the inner filter and the cylinder there is arranged a slide shoe, defining a dewatering shoe, having a curved or domed surface which is curved in the same sense as the cylinder and the radius of which is larger than that of the cylinder. The outer filter which is furthest from the cylinder is provided with an adjustable guide roll which, depending upon its position, enables displacement of the contact line at which the outer filter travels onto the inner filter.
A double filter papermaking machine equipped with a dewatering cylinder and a linear section of an inner filter, located forwardly of the dewatering cylinder, is disclosed in U.S. Pat. No. 3,201,305, granted Aug. 17, 1965 to David R. Webster, reference being specifically made to FIG. 2. With this prior art machine there should be attained a double face dewatering of the formed paper web, and specifically, along the linear section under the influence of the force of gravity and at the dewatering cylinder by the filter tension and the centrifugal force. This state-of-the-art machine has the advantage that guiding of the filters occurs with low friction, since there does not exist any friction between the filter and the rotating filter which revolves with the same peripheral speed. The dewatering effect at the web or fleece however occurs suddenly upon reaching the cylinder surface. This is associated with a series of drawbacks, for instance the formation of so-called needle or pin holes.
On the other hand, in U.S. Pat. No. 4,033,812, granted July 5, 1977, and assigned to the Finnish firm Valmet Oy, there is disclosed a double filter or twin-wire papermaking machine wherein the dewatering effect due to filter tension and centrifugal force predominantly occurs at a shoe over which slide the filters. The subsequent arranged cylinder only is assigned the task of post-drying the paper fleece or web. Additionally, there is provided an adjustable guide roll enabling a partial pivoting of the upper filter from the shoe. However, this guide roll only has the function of enabling displacement of a carriage with the outer filter from the inner filter. With this paper machine it is possible, by selecting the radii of the surface of the shoe, to influence the course and magnitude of the pressure forces of both filters acting upon the fiber web or fleece. The movement of the filter over a stationary shoe with the required large filter extent is however associated with the formation of large frictional forces and appreciable wear of the shoe and the filter. Fabrication of a shoe having different radii constitutes a decisive technological problem.